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Chemical Vapor Deposition of Carbon Nanotubes: A Review on Growth

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REVIEW

Copyright © 2010 American Scientific Publishers

All rights reserved

Printed in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 10, 3739–3758, 2010

Chemical Vapor Deposition of Carbon Nanotubes:

A Review on Growth Mechanism and Mass Production

Mukul Kumar∗ and Yoshinori AndoDepartment of Materials Science and Engineering, Meijo University, Nagoya 468-8502, Japan

This review article deals with the growth mechanism and mass production of carbon nanotubes(CNTs) by chemical vapor deposition (CVD). Different aspects of CNT synthesis and growth mech-anism are reviewed in the light of latest progresses and understandings in the field. Materialsaspects such as the roles of hydrocarbon, catalyst and catalyst support are discussed. Many newcatalysts and new carbon sources are described. Growth-control aspects such as the effects oftemperature, vapor pressure and catalyst concentration on CNT diameter distribution and single- ormulti-wall formation are explained. Latest reports of metal-catalyst-free CNT growth are considered.The mass-production aspect is discussed from the perspective of a sustainable CNT technology.Existing problems and challenges of the process are addressed with future directions.

Keywords: Chemical Vapor Deposition (CVD), Carbon Nanotube (CNT), CNT GrowthMechanism, CNT Mass Production, CNT Industrial Production, CNT Precursor, CNTCatalyst, Catalyst-Free CNT Synthesis, Camphor.

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3739

2. Chemical Vapor Deposition (CVD) . . . . . . . . . . . . . . . . . . . . . . 3740

2.1. History of CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3740

2.2. Advantages of CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3741

3. CNT Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3741

3.1. CNT Precursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3742

3.2. CNT Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3743

3.3. CNT Catalyst Supports . . . . . . . . . . . . . . . . . . . . . . . . . . . 3743

3.4. New (Uncommon) CNT Catalysts . . . . . . . . . . . . . . . . . . . 3744

3.5. Metal-Free CNT Growth . . . . . . . . . . . . . . . . . . . . . . . . . . 3744

3.6. New (Unconventional) CNT Precursors . . . . . . . . . . . . . . . 3745

4. CNT Growth Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3746

4.1. Effect of Catalyst Material and Concentration . . . . . . . . . . 3746

4.2. Effect of Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3747

4.3. Effect of Vapor Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . 3748

5. CNT Growth Mechanism Under Electron Microscopy . . . . . . . 3748

5.1. Physical State of the Catalyst . . . . . . . . . . . . . . . . . . . . . . . 3748

5.2. Mode of Carbon Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . 3749

5.3. Chemical State of the Catalyst . . . . . . . . . . . . . . . . . . . . . . 3751

6. Mass Production of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3751

6.1. Mass Production of CNTs from

Conventional Precursors . . . . . . . . . . . . . . . . . . . . . . . . . . . 3751

6.2. Mass Production of CNTs from Camphor . . . . . . . . . . . . . 3752

6.3. Camphor versus Conventional CNT Precursors . . . . . . . . . 3753

6.4. Environment-Friendliness of Camphor CVD . . . . . . . . . . . 3753

6.5. Industrial Production of CNTs from Camphor . . . . . . . . . . 3754

7. Existing Challenges and Future Directions . . . . . . . . . . . . . . . . 3754

∗Author to whom correspondence should be addressed.

8. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3754

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3755

References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3755

1. INTRODUCTION

A carbon nanotube (CNT) is a tubular structure made

of carbon atoms, having diameter of nanometer order

but length in micrometers. Although this kind of struc-

tures was synthesized, studied and reported by several

researchers during 1952–1989,1–17 Iijima’s detailed anal-

ysis of helical arrangement of carbon atoms on seam-

less coaxial cylinders in 1991, proved to be a discovery

report.18 Since then, CNT has remained an exciting mate-

rial ever. Its so-called extraordinary properties: many-fold

stronger than steel, harder than diamond, electrical con-

ductivity higher than copper, thermal conductivity higher

than diamond, etc. set off a gold rush in academic and

industrial laboratories all over the world to find practical

uses of CNTs. This sprouted thousands of publications and

patents on innumerous potential applications of CNTs in

almost all the walks of life: media, entertainment, commu-

nication, transport, health and environment. The gold rush

turned into a stampede when NASA scientists and many

others predicted the possibility of making space elevators,

lighter and stronger aircrafts, collapsible and reshapable

cars, incredible new fabric, portable X-ray machines etc.

J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 6 1533-4880/2010/10/3739/020 doi:10.1166/jnn.2010.2939 3739

Dr. Mukul Kumar

Typewriter

E-mail: [email protected]

REVIEW

Chemical Vapor Deposition of Carbon Nanotubes: A Review on Growth Mechanism and Mass Production Kumar and Ando

by using CNTs. Consequently, CNT has become a material

of common interest today; and society is eagerly waiting

for seeing the charisma of CNT in household products.

However, even after 18 years of continued efforts world-

wide, such products are still waitlisted.

The bottleneck is insufficient production and uncom-

petitive cost of CNTs with respect to the prevalent tech-

nology. Despite a huge progress in CNT research over

the years, we are still unable to produce CNTs of well-

defined properties in large quantities with a cost-effective

technique. The root of this problem is the lack of proper

understanding of the CNT growth mechanism. Till date

no definitive model could be robustly established for the

CNT growth. There are several issues in the growth mech-

anism that are yet to be clarified. Ironically, from the win-

dow of time machine, the CNT research today is ahead

of its time. We, the CNT researchers, know how to make

single-electron transistors from individual CNTs, but we

do not know how to make a CNT of the required structure.

Hence it is necessary to retrospect. The skytower of the

ambitious nanotechnology (in particular, CNT-based tech-

nology) cannot be erected without a firm foundation of the

growth-mechanism understanding.

Among several techniques of CNT synthesis available

today, chemical vapor deposition (CVD) is most popu-

lar and widely used because of its low set-up cost, high

production yield, and ease of scale-up. This review, there-

fore, deals with the growth mechanism and mass produc-

tion of CNTs by CVD. Beginning with a brief historical

account of CVD pertinent to CNT, we will cover differ-

ent aspects of CNT synthesis and growth mechanism in

Dr. Mukul Kumar received his B.Sc., M.Sc. and Ph.D. degrees in the Faculty of Sci-

ence (Physics) from Bihar University, India. During 1990–1996, he served as a Lecturer

of Physics in Jai Prakash University, India. In 1996, he joined Indian Institute of Tech-

nology, Bombay and worked for Indo-French, UNESCO, DMSRDE and CSIR Projects. In

Dec 2000, he joined Meijo University as a JSPS Fellow, and since Jan 2003, he has been a

senior scientist at the 21st Century Centre of Excellence there. Actively involved in research,

research guidance and education, Dr. Kumar’s areas of interest include electrochemistry

and photo-electrochemistry of group II–VI semiconductors, synthesis, characterization and

application of CNTs grown by CVD method, and electron field emission. He has published

48 research articles and 6 patents on syntheses of glassy carbon and carbon nanotubes, and

presented 80 papers at international conferences.

Professor Yoshinori Ando received his B.E., M.E. and D.E. degrees in the Faculty of

Engineering (Applied Physics) from Nagoya University, Japan. After five years post-doctoral

experience in Nagoya University, he moved to Meijo University as a Lecturer in Physics in

1974. He became Assistant Professor in 1977, and Professor in 1990. During 1987–1988, he

was a visiting research fellow at Bristol University, UK. During 2000–2004, he was the first

Head of the Department of Materials Science and Engineering in Meijo University. From

April 2009, he is the Dean of the Faculty of Science and Technology in Meijo University.

Professor Ando’s research areas include X-ray diffraction topography of distorted crystals,

electron microscopy of thin films and ultrafine particles of SiC, arc-discharge synthesis of

CNTs and their application to composite materials. He has published over 200 research

articles and a dozen of patents. He has presented over 100 papers at various conferences.

the light of latest progresses and understandings in the

field. On the materials aspect, the roles of hydrocarbon

and catalyst are discussed in detail. Many new catalysts

and new carbon sources are addressed. Among the new

CNT precursors, camphor is highlighted because of its

environment-friendliness and exceptionally-high efficiency

toward CNT production. On the growth-control aspect, the

effects of temperature, vapor pressure and catalyst concen-

tration on diameter distribution and single- or multi-wall

formation are discussed. The mass-production aspect is

covered stressing the need of a sustainable CNT technol-

ogy. Finally, we conclude with a brief mention of existing

challenges and future directions.

2. CHEMICAL VAPOR DEPOSITION (CVD)

Chemical vapor deposition (CVD) is the most popular

method of producing CNTs nowadays. In this process,

thermal decomposition of a hydrocarbon vapor is achieved

in the presence of a metal catalyst. Hence, it is also known

as thermal CVD or catalytic CVD (to distinguish it from

many other kinds of CVD used for various purposes).

2.1. History of CVD

The history of CVD for the synthesis of carbon filaments

dates back to nineteenth century. In 1890, French scientists

observed the formation of carbon filaments during exper-

iments involving the passage of cyanogens over red-hot

porcelain.19 By mid-twentieth century, CVD was an estab-

lished method for producing carbon microfibers utilizing

3740 J. Nanosci. Nanotechnol. 10, 3739–3758, 2010

REVIEW

Kumar and Ando Chemical Vapor Deposition of Carbon Nanotubes: A Review on Growth Mechanism and Mass Production

thermal decomposition of hydrocarbons in the presence of

metal catalysts. In 1952 Radushkevich and Lukyanovich

published a range of electron micrographs clearly exhibit-

ing tubular carbon filaments of 50–100 nm diameter grown

from thermal decomposition of carbon monoxide on iron

catalyst at 600 �C.1 They observed iron carbides encapsu-

lated in the filament tips; accordingly, they proposed that,

at first, carbon dissolution in iron resulted in the forma-

tion of iron carbide, and then, subsequent carbon depo-

sition over iron carbide led to the formation of graphene

layers. In the same year, another Russian group, Tes-

ner and Echeistova, also reported similar carbon threads

on lampblack particles exposed to methane, benzene or

cyclohexane atmospheres at temperatures above 977 �C.2

In 1953, Davis et al. published detailed electron micro-

graphs and XRD spectra of carbon nanofibers grown from

the reaction of CO and Fe2O4 at 450 �C in blast fur-

nace brickworks.3 They postulated that the catalyst for the

reaction, either iron or iron carbide, formed on the sur-

face of the iron oxide as a speck which in turn gave rise

to a thread of carbon. They suggested that, at the time

of carbon deposition, the catalyst particles were located

on the growing ends of the threads. The threads were

described as layered carbon, varying in thickness from

10 to 200 nm. Similar findings were reported by Hofer

et al. (1955),4 Walker (1959)5 and Baird et al. (1971).6�7 In

the 1970s extensive works were carried out independently

by Baker and Endo to synthesize and understand tubular

nanofibers of multi-layered carbon.8–12 Thus, today’s most-

popular CNT technique, the CVD, may probably be the

most-ancient technique of growing CNTs in the name of

filaments and fibers. More detailed reviews of the early

works have been written by Baker,13�14 Endo,15�20 and

Dresselhaus.16�17

2.2. Advantages of CVD

As compared to arc-discharge and laser-ablation methods,

CVD is a simple and economic technique for synthesiz-

ing CNTs at low temperature and ambient pressure. In

crystallinity, arc- and laser-grown CNTs are superior to

the CVD-grown ones (although CVD-grown MWCNTs

possess inferior crystallinity, the crystallinity of SWCNTs

grown by CVD is close to that grown by arc or laser meth-

ods). However, in yield and purity, CVD beats the arc and

laser methods. And, when it comes to structure control

or CNT architecture, CVD is the only answer. CVD is

versatile in the sense that it offers harnessing plenty of

hydrocarbons in any state (solid, liquid or gas), enables

the use of various substrates, and allows CNT growth in

a variety of forms, such as powder, thin or thick films,

aligned or entangled, straight or coiled nanotubes, or a

desired architecture of nanotubes on predefined sites of

a patterned substrate. It also offers better control on the

growth parameters.

3. CNT SYNTHESIS

Figure 1 shows a schematic diagram of the experimen-

tal set-up used for CNT growth by CVD method in its

simplest form. The process involves passing a hydrocar-

bon vapor (typically 15–60 min) through a tubular reac-

tor in which a catalyst material is present at sufficiently

high temperature (600–1200 �C) to decompose the hydro-

carbon. CNTs grow on the catalyst in the reactor, which

are collected upon cooling the system to room tempera-

ture. In the case of a liquid hydrocarbon (benzene, alco-

hol, etc.), the liquid is heated in a flask and an inert gas

is purged through it, which in turn carries the hydrocar-

bon vapor into the reaction zone. If a solid hydrocarbon is

to be used as the CNT precursor, it can be directly kept

in the low-temperature zone of the reaction tube. Volatile

materials (camphor, naphthalene, ferrocence etc.) directly

turn from solid to vapor, and perform CVD while pass-

ing over the catalyst kept in the high-temperature zone.

Like the CNT precursors, also the catalyst precursors in

CVD may be used in any form: solid, liquid or gas, which

may be suitably placed inside the reactor or fed from out-

side. Pyrolysis of the catalyst vapor at a suitable tempera-

ture liberates metal nanoparticles in-situ (such a process is

known as floating catalyst method). Alternatively, catalyst-

coated substrates can be placed in the hot zone of the

furnace to catalyze the CNT growth.

CNT growth mechanism has been debatable right from

its discovery. Based on the reaction conditions and post-

deposition product analyses, several groups have pro-

posed several possibilities which are often contradicting.

Therefore, no single CNT growth mechanism is well

established till date. Nevertheless, widely-accepted most-

general mechanism can be outlined as follows. Hydrocar-

bon vapor when comes in contact with the “hot” metal

nanoparticles, first decomposes into carbon and hydrogen

species; hydrogen flies away and carbon gets dissolved

into the metal. After reaching the carbon-solubility limit

in the metal at that temperature, as-dissolved carbon pre-

cipitates out and crystallizes in the form of a cylindrical

network having no dangling bonds and hence energetically

stable. Hydrocarbon decomposition (being an exother-

mic process) releases some heat to the metal’s exposed

Fig. 1. Schematic diagram of a CVD setup in its simplest form.

J. Nanosci. Nanotechnol. 10, 3739–3758, 2010 3741

REVIEW


zone, while carbon crystallization (being an endothermic

process) absorbs some heat from the metal’s precipitation

zone. This precise thermal gradient inside the metal parti-

cle keeps the process on.

Now there are two general cases. (Fig. 2(a)) When the

catalyst–substrate interaction is weak (metal has an acute

contact angle with the substrate), hydrocarbon decomposes

on the top surface of the metal, carbon diffuses down

through the metal, and CNT precipitates out across the

metal bottom, pushing the whole metal particle off the

substrate (as depicted in step (i)). As long as the metal’s

top is open for fresh hydrocarbon decomposition (concen-

tration gradient exists in the metal allowing carbon diffu-

sion), CNT continues to grow longer and longer (ii). Once

the metal is fully covered with excess carbon, its catalytic

activity ceases and the CNT growth is stopped (iii). This

is known as “tip-growth model.”8

In the other case, (Fig. 2(b)) when the catalyst–substrate

interaction is strong (metal has an obtuse contact angle

with the substrate), initial hydrocarbon decomposition and

carbon diffusion take place similar to that in the tip-growth

case, but the CNT precipitation fails to push the metal par-

ticle up; so the precipitation is compelled to emerge out

from the metal’s apex (farthest from the substrate, hav-

ing minimum interaction with the substrate). First, carbon

crystallizes out as a hemispherical dome (the most favor-

able closed-carbon network on a spherical nanoparticle)

which then extends up in the form of seamless graphitic

(a)

(b)

Fig. 2. Widely-accepted growth mechanisms for CNTs: (a) tip-growth model, (b) base-growth model.

cylinder. Subsequent hydrocarbon deposition takes place

on the lower peripheral surface of the metal, and as-

dissolved carbon diffuses upward. Thus CNT grows up

with the catalyst particle rooted on its base; hence, this is

known as “base-growth model.”10

Formation of single- or multi-wall CNT (SWCNT or

MWCNT, respectively) is governed by the size of the cata-

lyst particle.21 Broadly speaking, when the particle size is a

few nm, SWCNT forms; whereas particles—a few tens nm

wide—favor MWCNT formation. With such an approx-

imate growth picture in mind, we can proceed to other

important aspects of the CNT growth. Detailed discussion

on the growth mechanism is presented in Section 5.

CNT synthesis involves many parameters such as hydro-

carbon, catalyst, temperature, pressure, gas-flow rate,

deposition time, reactor geometry. However, to keep our

discussion compact, here we will consider only the three

key parameters: hydrocarbon, catalyst and catalyst support.

3.1. CNT Precursors

Most commonly used CNT precursors are methane,22�23

ethylene,24�25 acetylene,26 benzene,27 xylene,28 and car-

bon monoxide.29 Endo et al.30–32 reported CNT growth

from pyrolysis of benzene at 1100 �C, whereas Jose-

Yacaman et al.33 got clear helical MWCNTs at 700 �Cfrom acetylene. In those cases iron nanoparticles were

used as the catalyst. Later, MWCNTs were also grown


REVIEW


from many other precursors including cyclohexane,34�35

and fullerene.36�37 On the other hand, SWCNTs were first

produced by Dai et al.38 from disproportionation of car-

bon monoxide at 1200 �C, in the presence of molybde-

num nanoparticles. Later, SWCNTs were also produced

from benzene,39 acetylene,40 ethylene,41 methane,23�42

cyclohexane,43 fullerene44 etc. by using various catalysts.

In 2002 Maruyama et al. reported the low-temperature

synthesis of high-purity SWCNTs from alcohol on

Fe–Co-impregnated zeolite support;45 and since then,

ethanol became the most popular CNT precursor in the

CVD method worldwide.46–48 Special feature of ethanol

is that ethanol-grown CNTs are almost free from amor-

phous carbon, owing to the etching effect of OH radical.49

Later, vertically-aligned SWCNTs were also grown on

Mo-Co-coated quartz and silicon substrates.50�51 Recently,

Maruyama’s group has shown that intermittent supply of

acetylene in ethanol CVD significantly assists ethanol in

preserving the catalyst’s activity and thus enhances the

CNT growth rate.52

The molecular structure of the precursor has a detri-

mental effect on the morphology of the CNTs grown.

Linear hydrocarbons such as methane, ethylene, acety-

lene, thermally decompose into atomic carbons or linear

dimers/trimers of carbon, and generally produce straight

hollow CNTs. On the other hand, cyclic hydrocarbons such

as benzene, xylene, cyclohexane, fullerene, produce rel-

atively curved/hunched CNTs with the tube walls often

bridged inside.36�37

General experience is that low-temperature CVD

(600–900 �C) yields MWCNTs, whereas high-temperature

(900–1200 �C) reaction favors SWCNT growth. This indi-

cates that SWCNTs have a higher energy of formation

(presumably owing to small diameters; high curvature

bears high strain energy). Perhaps that is why MWCNTs

are easier to grow (than SWCNTs) from most of the hydro-

carbons, while SWCNTs grow from selected hydrocarbons

(viz. carbon monoxide, methane, etc. which have a reason-

able stability in the temperature range of 900–1200 �C).Commonly efficient precursors of MWCNTs (viz. acety-

lene, benzene, etc.) are unstable at higher temperature and

lead to the deposition of large amounts of carbonaceous

compounds other than the nanotubes.

In 2004, using ethylene CVD, Hata et al. reported

water-assisted highly-efficient synthesis of impurity-free

SWCNTs on Si substrates.53 They proposed that con-

trolled supply of steam into the CVD reactor acted as a

weak oxidizer and selectively removed amorphous carbon

without damaging the growing CNTs. Balancing the rel-

ative levels of ethylene and water was crucial to maxi-

mize the catalyst’s lifetime. However, very recently, Zhong

et al. have shown that a reactive etchant such as water or

hydroxyl radical is not required at all in cold-wall CVD

reactors if the hydrocarbon activity is low.54 These studies

emphatically prove that the carbon precursor plays a cru-

cial role in CNT growth. Therefore, by proper selection of

CNT precursor and its vapour pressure, both the catalyst’s

lifetime and the CNT-growth rate can be significantly

increased; and consequently, both the yield and the quality

of CNTs can be improved.

3.2. CNT Catalysts

For synthesizing CNTs, typically, nanometer-size metal

particles are required to enable hydrocarbon decomposition

at a lower temperature than the spontaneous decomposi-

tion temperature of the hydrocarbon. Most commonly-used

metals are Fe, Co, Ni, because of two main reasons: (i) high

solubility of carbon in these metals at high temperatures;

and (ii) high carbon diffusion rate in these metals. Besides

that, high melting point and low equilibrium-vapor pres-

sure of these metals offer a wide temperature window of

CVD for a wide range of carbon precursors. Recent con-

siderations are that Fe, Co, and Ni have stronger adhesion

with the growing CNTs (than other transition metals do)

and hence they are more efficient in forming high-curvature

(low-diameter) CNTs such as SWCNTs.55

Solid organometallocenes (ferrocene, cobaltocene, nick-

elocene) are also widely used as a CNT catalyst, because

they liberate metal nanoparticles in-situ which catalyze the

hydrocarbon decomposition more efficiently. It is a gen-

eral experience that the catalyst-particle size dictates the

tube diameter. Campbell’s group has reported the particle-

size dependence and a model for iron-catalyzed growth

of CNTs.37 Hence, metal nanoparticles of controlled size,

pre-synthesized by other reliable techniques, can be used

to grow CNTs of controlled diameter.56 Thin films of cat-

alyst coated on various substrates are also proven good

in getting uniform CNT deposits.24 The key factor to get

pure CNTs is achieving hydrocarbon decomposition on the

catalyst surface alone, and prohibiting the aerial pyrolysis.

Apart from the popular transition metals (Fe, Co, Ni),

other metals of this group, such as Cu, Au, Ag, Pt, Pd

were also found to catalyze various hydrocarbons for CNT

growth. A comprehensive review of the role of metal par-

ticles in the catalytic growth of CNTs has been published

by Kauppinen’s group.57

On the role of CNT catalysts, it is worth mention-

ing that transition metals are proven to be efficient cat-

alysts not only in CVD but also in arc-discharge and

laser-vaporization methods. Therefore, it is likely that

these apparently different methods might inherit a com-

mon growth mechanism of CNT, which is not yet clear.

Hence this is an open field of research to correlate dif-

ferent CNT techniques in terms of the catalyst’s role in

entirely different temperature and pressure range.

3.3. CNT Catalyst Supports

The same catalyst works differently on different support

materials. Commonly used substrates in CVD are

graphite,8�9 quartz,58�59 silicon,53�60 silicon carbide,61�62


REVIEW


silica,63�64 alumina,65�66 alumino-silicate (zeolite),67�68

CaCO3,69 magnesium oxide,70–72 etc. For an efficient CNT

growth, the catalyst–substrate interaction should be inves-

tigated with utmost attention. Metal–substrate reaction

(chemical bond formation) would cease the catalytic behav-

ior of the metal. The substrate material, its surface mor-

phology and textural properties greatly affect the yield and

quality of the resulting CNTs. Zeolite supports with cat-

alysts in their nanopores have resulted significantly high

yields of CNTs with a narrow diameter distribution.68�73

Alumina materials are reported to be a better catalyst sup-

port than silica owing to the strong metal–support inter-

action in the former, which allows high metal dispersion

and thus a high density of catalytic sites.74 Such interac-

tions prevent metal species from aggregating and forming

unwanted large clusters that lead to graphite particles or

defective MWCNTs.75 Recent in-situ XPS analysis of CNT

growth from different precursors on iron catalyst supported

on alumina and silica substrates have confirmed these the-

oretical assumptions.64 Thin Alumina flakes (0.04–4 �mthick) loaded with iron nanoparticles have shown high

yields of aligned CNTs of high aspect ratio.76 Latest con-

siderations are that the oxide substrate, basically used as

a physical support for the metal catalyst, might be play-

ing some chemistry in the CNT growth.77 Acoordingly,

the chemical state and structure of the substrate are more

important than that of the metal.

3.4. New (Uncommon) CNT Catalysts

Recent developments in the nanomaterials synthesis and

characterization have enabled many new catalysts for the

CNT growth. Apart from popularly used transition met-

als (Fe, Co, Ni), a range of other metals (Cu, Pt, Pd,

Mn, Mo, Cr, Sn, Au, Mg, Al) has also been successfully

used for horizontally-aligned SWCNT growth on quartz

substrates.78 Unlike transition metals, noble metals (Au,

Ag, Pt, Pd etc.) have extremely low solubility for car-

bon, but they can dissolve carbon effectively for CNT

growth when their particle size is very small (<5 nm).79�80

Daisuke et al. have succeeded in controlled growth of

SWCNTs on Au nanoparticles deposited on atomic steps

of Si. It has been proposed that the active catalyst is Au–Si

alloy with about 80 at% Au.71

Although copper is a transition metal, it showed insignif-

icant catalytic effect on the CNT growth in the past.82–85 In

fact, it had been considered as an adverse contaminant. As a

recent development, however, it has been found to catalyze

the CNT growth efficiently. Methane and ethanol decom-

position at 825–925 �C on Cu nanoparticles supported on

silicon wafers produced high densities of well-crystallined

SWCNTs up to 1 cm in length.86 The Cu nanoparticles

were synthesized by the reduction of CuCl2 in the pres-

ence of Cu2O nanoparticles produced by the thermolysis

of cupric formates in coordinating solvents.87 This implies

that the novelty lies in the catalyst-preparation method.

Rhenium (Re) is a rare catalyst used for CNT growth.

Diamagnetic SWCNTs and MWCNTs were reported by

methane decomposition on Re catalyst.88 The current sce-

nario is that, just as any carbon-containing material can

yield CNT, any metal can catalyze the CNT growth,

provided that the experimental conditions are properly

optimized.

3.5. Metal-Free CNT Growth

Very recently nanodiamond particles (5 nm) were suc-

cessfully used as a CNT catalyst. Ethanol suspension of

nanodiamond particles was spread on graphite plates and

dried in air at 600 �C. This resulted in isolated dia-

mond particles, monolayers of diamond, and multilayered-

diamond stacks on the substrate, depending upon the dia-

mond concentration (0.01–1.0 wt%). Ethanol CVD over

these diamond-loaded substrates at 850 �C produced iso-

lated CNTs, layered CNTs and high-density CNT mats,

respectively. The nanodiamond particles do not fuse even

after high-temperature CVD process, implying that they

remain in solid state during CVD. Nanodiamond is there-

fore said to act as a CNT growth seed.89 This result proves

that CNT growth is possible without metal catalyst. Does

nanodiamond act as a catalyst? If it does, how? These are

open questions.

In many studies, oxygen was noticed to activate the CNT

growth. Recent studies have revealed that many metals,

which do not exhibit catalytic activity in pure-metal form,

do well in oxide form.90 Does metal oxide act as a cata-

lyst? Template-free directional growth of CNTs has been

achieved on sapphire.91 CNTs have also been grown on

semiconductors such as Si and Ge nanoparticles (though

C has little solubility in bulk Si or Ge), provided that the

nanoparticles are heated in air just before CVD.92 Simi-

larly, CNT growth on SiC substrates takes place only when

some oxygen is present in the chamber.93 Porous Al2O3

has already been shown to facilitate CNT growth without

any catalyst.94�95 Catalyst-free CNT growth is also possible

in oxy-fuel flames.96 And oxide, typically used as a cata-

lyst support in CVD, is itself capable of forming graphene

layers.97 All these examples resoundingly indicate that oxy-

gen plays a key role in CNT growth. The question is: is

it a catalyst? HRTEM investigation of the CNTs grown

by cyclohexane pyrolysis over iron nanoparticles supported

on thin Al2O3 layers shows that CNT keeps on growing

even when the metal is completely encapsulated in the tube

center.98 The authors propose that the metal only helps to

initiate the CNT precipitation at the nucleation stage. Once

the CNT head is created, metal becomes non functional;

subsequent carbon addition to the CNT base periphery is

facilitated by the substrate’s oxide layer. This concept is

radically different to the existing concept that the metal

must remain exposed (either on the CNT tip or base) to

keep the growth on. Hence more careful in-situ observation


REVIEW


and robust theoretical support are required to establish the

oxide’s direct role as a catalyst.

The latest development of the field is even more excit-

ing. CNT growth is possible with no metal at all; the

non-metallic substrate itself acts as the catalyst. Liu et al.

passed methane and hydrogen (1:1) over an SiO2-sputtered

Si wafer at 900 �C for 20 min and got dense SWCNTs

grown on it. In the same CVD condition, thermally-grown

SiO2 films did not result CNTs. The success lies in the

in-situ transformation of the sputtered SiO2 film (30 nm)

into isolated Si particles (1.9 nm) which efficiently cat-

alyzed methane decomposition due to small-size effect.99

Similar SiO2 nanoparticle generation and subsequent CNT

growth was reported by Liu et al. from ethanol decom-

position on annealed SiO2/Si substrates.100 On the other

hand, Huang et al. simply scratched the existing SiO2/Si

wafers by a diamond blade and passed ethanol over it

at 900 �C for 10 min.101 Bunch of SWCNTs grew on

the scratched portions. Random scratches on thin SiO2

films protrude some nanoparticles mechanically. These

developments raise many new questions and compel us

to reconsider the existing CNT-growth models. SiO2 has

no carbon solubility; how does it assist hydrocarbon-to-

CNT conversion? Does it act as a solid-state catalyst like

nanodiamond? Or does it melt at 900 �C as usual metal

nanoparticles do? If it is in molten state, Si and O atoms

might have some mobility thus creating a vacancy or dis-

location which would attract hydrocarbon and cause dehy-

drogenation. If it is in solid state, it would be strained

enough (high curvature at small particle size) and could

possibly interact with hydrocarbon. It is also likely that

it is in a quasi-liquid (fluctuating solid) state; slight dis-

tortion in its overall shape would develop some polar-

ity on Si and O atoms, which could possibly facilitate

dehydrogenation; and its fluctuating shape could act as

a template for tubular graphite formation. These specu-

lations evoke serious discussion. SiO2 has a number of

distinct crystalline forms. Si–O bond length and Si–O–Si

bond angle vary significantly in different crystal forms

(e.g., 154–171 pm, 140�–180�). Ab-initio calculations have

indicated that CNT-cap nucleation is influenced by solid-

surface curvatures.102 More theoretical considerations and

experimental verifications are sought for proper under-

standing of CNT growth on SiO2 nanoparticles. It will take

due time to come up with a convincing model; neverthe-

less, there is no doubt that metal-free CNT synthesis is a

major breakthrough in CNT research, and it has opened

up a new avenue in nanotechnology.

3.6. New (Unconventional) CNT Precursors

Apart from the popular hydrocarbons mentioned in

Section 3.1, CNTs have also been synthesized from

many other organic compounds, especially from poly-

mers. Carbonization (prolonged pyrolysis in vacuum

to convert organic compounds into solid carbon) of

polyacrylonitrile103 and poly-furfuryl-alcohol104 within

nanoporous alumina templates resulted in thick CNTs.

Reported in 1995–96, this was a multi-step tedious pro-

cess requiring chemically-controlled monomer initiators

to achieve polymerization. The field has matured enough

and nowadays super-aligned highly-uniform CNTs can be

produced from readily available polymers without taking

pains for chemical initiators or catalysts. Recently, sev-

eral polymer precursors, loaded on commercially-available

alumina templates of well-defined pore size, were car-

bonized (400–600 �C for 3 h) to obtain MWCNTs of

desired diameter.105 N-doped MWCNTs obtained from

carbonization of polypyrrole within alumina and zeolite

membranes have shown better hydrogen-storage capacity

than pristine MWCNTs obtained from polyphenyl acety-

lene in the same conditions.106 As for SWCNT, pyroly-

sis of tripropylamine within the nanochannels (0.73 nm)

of aluminophosphate crystals (AFI) resulted in the small-

est nanotubes (0.4 nm).107 Later, several carbon precursors

were pyrolyzed within the AFI channels and tetrapropy-

lammonium hydroxide was found to yield high densities

of 4Å CNTs with better crystallinity.108 It is suggested

that the number of carbon atoms in the precursor molecule

influences the SWCNT packing density in the template

channels.

Among other organic compounds, amino-dichloro-s-

triazine, pyrolyzed on cobalt-patterned silica substrates,

resulted in highly pure CNTs.109 Almost contemporary,

organometallic compounds such as metallocene (ferrocene,

cobaltocene, nickelocene)27 and nickel phthalocyanine110

were used as the carbon-cum-catalyst precursor; however,

as-grown CNTs were highly metal-encapsulated and the

yield was very low. Later, pyrolysis of thiophene with met-

allocene led to the formation of Y -junction CNTs.111�112

Recently, high-temperature pyrolysis (1300 �C) of simple

saccharides (from table sugar (sucrose) to lactose) resulted

in straight as well as helical MWCNTs.113

In 2001 high yield of CNTs was obtained from camphor,

a tree product.114 Since then the authors remained involved

with this environment-friendly source of CNTs and

established the conditions for growing MWCNTs,115�116

SWCNTs,117 and vertically-aligned CNTs on quartz and

silicon substrates59�60�118 using ferrocene catalyst. Later,

using Fe–Co catalyst impregnated in zeolite support,

mass production of CNTs was achieved by camphor

CVD.68�119 MWCNTs were grown at a temperature as low

as 550 �C, whereas SWCNTs could be grown at rela-

tively high (900 �C) temperature. Because of very low cat-

alyst requirement with camphor, as-grown CNTs are least

contaminated with metal, whereas oxygen atom present

in camphor helps in oxidizing amorphous carbon in-situ.These features of camphor stimulated more in-depth, basic

and applied research worldwide. Collaboration of India,

UK and Japan reported the use of camphor-grown CNTs


REVIEW


as the anode of secondary lithium battery.120 Scientists

at the University of Edinburgh investigated the effect of

camphor’s molecular structure on the CNT growth and

quality.121 Researchers at Tokyo University of Science

studied camphor CVD with different ways of catalyst feed-

ing and addressed catalyst activation/deactivation process

for the synthesis of highly-dense aligned CNT arrays.122

Another Japanese group synthesized aligned CNTs from

camphor and reported their field emission properties.123�124

South African scholars studied the effect of carrier gases

(nitrogen, argon, argon–hydrogen mixture) as well as

catalyst–support materials (SiO2, Al2O3 and MgO) on the

quality of camphor-grown CNTs.125 Brazilian scientists

carried out thermal annealing and electrochemical purifi-

cation of camphor-grown CNTs.126 Chinese carbon group

synthesized tree-like multi-branched CNTs from camphor

and reported the effects of temperature, argon flow rate and

catalyst concentration on the structure of as-grown carbon

nanotrees.127 Italian carbon group investigated different

aspects of camphor-CVD and published a series of reports.

Musso et al. achieved low-temperature (650 �C) growth

of vertically-aligned CNTs on various glass substrates and

confirmed the high CNT-growth efficiency of camphor.128

Porro et al. optimized the CNT growth parameters by

Taguchi method and got 2.3 mm thick CNT mats at a

high deposition rate of 500 nm/sec.129�130 Later, the same

group published fluid-dynamic analysis of the carrier-gas

flow for camphor-CVD system131�132 and hydrogen-storage

analysis of different kinds of CNTs.133 Thus, camphor has

emerged as a promising and the most-efficient CNT pre-

cursor amongst the new/unconventional ones. Moreover, it

has opened up a new avenue of exploring other botani-

cal products as a CNT precursor. Appreciable efforts have

been made in India by Sharon and his coworkers who

investigated the pyrolysis of a range of plant-based mate-

rials for this purpose.134 Recently, high yields of aligned

and non-aligned CNTs have also been reported from other

plant-derived cheap raw materials such as turpentine135–137

and eucalyptus oils.138

Apart from the well-defined chemical reagents described

above, CNTs have also been successfully and systemati-

cally synthesized from domestic fuels such as kerosene,139

coal gas,140 and liquefied petroleum gas.141 More interest-

ingly, there are scientific reports of CNT production from

green grasses. Grass contains dense vascular bundles in

the stem and branches. They are mainly composed of cel-

lulose, hemicellulose and lignin. Rapid heat treatment of

grass (600 �C) in controlled-oxygen ambience dehydrates

and carbonizes the vascular bundles into CNTs.142 Thus,

now it is almost certain that any carbon-containing mate-

rial may be a CNT precursor under suitable experimental

conditions. The point is: can we reproduce the product

quality and quantity from those materials of inconsistent

composition? Certainly not. Depending upon the chemical

composition of the raw material, one will have to change

the experimental conditions every now and then; and the

impurity elements of the raw material would greatly con-

taminate the resulting CNTs which would ultimately be of

no practical importance. Hence, these novel and interest-

ing researches of CNT production from garbage materials

seem to be too long-term project. To meet the immediate

need of mass production of CNTs, it is advisable to choose

a raw material of consistent chemistry, which is highly

abundant and regenerative too; so that it could lead to a

reproducible as well as sustainable industrial technique.

4. CNT GROWTH CONTROL

It is well known that hydrocarbons are easily broken at

high temperatures. Such a thermal decomposition is called

Pyrolysis. However, in the presence of suitable metal cat-

alysts, a hydrocarbon can be decomposed at lower tem-

peratures (catalytic pyrolysis). The key of CNT growth by

CVD is to achieve the hydrocarbon decomposition on the

metal surface alone and prohibit spontaneous aerial pyrol-

ysis (do not allow the hydrocarbon to break uncatalyzed,

beyond the catalyst surface). Restriction of pyrolysis to

the catalyst surface is controlled through proper selection

of hydrocarbon and catalyst materials, vapor pressure of

the hydrocarbon, concentration of the catalyst, and the

CVD reaction temperature. Choosing camphor as the car-

bon source, the rest three key parameters were categori-

cally optimized in our laboratory.

4.1. Effect of Catalyst Material and Concentration

Early experiments of CNTs were carried out with dif-

ferent metal catalysts (Fe, Co, Ni) separately. Iron was

found to have high catalytic effect in hydrocarbon decom-

position leading to higher CNT deposits, but those CNTs

were poorly graphitized.22 On the other hand, cobalt cata-

lyst resulted in better-graphitized CNTs but the yield went

down.73 Hence a mixture of the two metals were tried to

combine their individual advantages, and it was success-

ful. Large volumes of well-graphitized MWCNTs were

obtained. Additional advantage of using the bimetallic cat-

alyst was that CNTs could be grown at much lower tem-

perature viz. 550 �C. It is because the melting point of the

mixture of Fe and Co is lower than their individual melting

points. Moreover, alloys are known to be better catalysts

than pure metals. These trends suggest that tri-metallic cat-

alysts should also give interesting results, though the inter-

pretation of result would be more complicated. No much

effort is known in this direction.

Besides the catalyst material, the catalyst concentration

also plays an important role in the CNT growth. In our lab,

catalyst concentration in zeolite was varied in a wide range

(1–50 wt%). No CNT formed for a catalyst concentration

less than 2.4 wt% in zeolite. Lower catalyst concentrations

(2.4–5%) exhibited SWCNT growth (at 850 �C and above),


REVIEW


whereas higher concentrations favored MWCNT growth.

A combined Fe+ Co concentration of 40% accounted

for the highest yield of MWCNTs with negligible metal

contamination.119 These studies confirm that SWCNTs or

MWCNTs can be selectively grown by proper selection of

catalyst materials and their concentration.

4.2. Effect of Temperature

The authors investigated the effect of temperature on cam-

phor CVD in a wide range of 500–1000 �C. It was noticedthat camphor did not decompose below 500 �C. At 550 �Cvery short-length tubes emerged from the zeolite pores

suggesting that the catalyst activity, and hence the CNT

growth rate, was quite low at 550 �C (Fig. 3(a)). How-

ever, the CNT growth abruptly increased at 600 �C, anda profound growth was observed all around the zeolite

pores (Fig. 3(b)). At 650�C and above, the growth rate

was so enormous that hardly a zeolite particle could be

located amid nanotubes. TEM investigations revealed that

the CNT diameter increased with the increasing growth

temperature. Very clean CNTs, almost free from metal-

lic impurity, were produced up to 750 �C. This suggests

that, at low temperatures, the catalyst–support interaction is

strong enough not to let the metal particles leave the zeolite

(a) (b)

20 nm 40 nm

60 nm

(c) (d)

Fig. 3. TEM images of MWCNTs grown at low temperatures and atmospheric pressure: (a) 550 �C, (b) 600 �C, (c) 900 �C, (d) SWCNTs at 900 �C.Reprinted with permission from [68], M. Kumar and Y. Ando, Carbon 43, 533 (2005). © 2005, Elsevier Science.

matrix. So, the CNTs grow via base-growth process. How-

ever, at higher temperatures, the metal particles were grad-

ually pronounced in the samples, preferably encapsulated

at the CNT tips. This suggests that, at critical temperatures,

tip-growth process coexists with base-growth process, due

to inhomogenity of metal dispersion. Bigger metal clus-

ters may leave a part of it to be lifted up with the growing

CNTs.

From 750 �C onward, both the diameter and the

diameter-distribution range increased drastically. It is

supposed that, at high temperature, the metal atoms

agglomerate into bigger clusters leading to thick CNTs.

Simultaneously; high temperature promotes camphor

decomposition leading to more carbon generation and

hence more wall formation. At 850 �C and above, SWC-

NTs began to form besides MWCNTs and the volume

of SWCNTs increased with the increasing temperature.

For instance, in 900 �C sample (Fig. 3(c)) large bun-

dles of SWNTs can be seen (in low contrast) under-

neath thick fibers (in high contrast). It is noteworthy that,

while MWCNTs and fibers have a lot of metal contam-

ination, there is no trace of metal associated with the

SWCNT bundles (Figs. 3(c, d)). Thus, the best temper-

ature for a gigantic growth of MWCNTs, was found to

be 650 �C, as optimized for Fe–Co-impregnated zeolite


REVIEW


catalyst. CNTs grown at such a low temperature were

almost free from metal particles as well as amorphous car-

bon. On the other hand, the best temperature for SWCNT

growth from camphor on Fe–Co–zeolite system was noted

to be 900 �C, restricting the total metal concentration (Fe+Co) to 2.4 wt% in zeolite (Fig. 3(d)). In such a low concen-

tration, the metal content remains confined to the zeolite

host even at 900 �C, and does not contaminate the growing

SWCNTs. Thus, the CVD temperature plays the central

role in CNT growth. For a fixed metal concentration, the

increasing CVD temperature increases the diameter distri-

bution. On the other hand, MWCNTs and SWCNTs can

be selectively grown as a function of CVD temperature,

if the catalyst’s concentration is properly optimized.68 A

good account of the effect of temperature on the growth

and structure of CNTs is reported by Li et al.143

4.3. Effect of Vapor Pressure

For controlled growth of CNTs by CVD, the vapor pres-

sure of the hydrocarbon in the reaction zone is another

very important parameter. For gaseous hydrocarbons, a

desired vapor pressure in the CVD reactor can be main-

tained by limited gas-flow rate and controlled suction with

a rotary pump.144 In the case of a liquid hydrocarbon,

its vapor pressure is controlled by its heating tempera-

ture before it enters the reactor.145 However, for a solid

hydrocarbon such as camphor, it is quite tricky to con-

trol its vapor pressure. It becomes a function of three

parameters: camphor mass, its vaporization temperature,

and the flow rate of argon—the carrier gas. By proper

optimization of these three parameters, the influx of cam-

phor vapor to the zeolite bed and its decomposition rate

were balanced to a great extent, and a record growth of

MWCNTs was achieved at atmospheric-pressure CVD.119

On the other hand, pure SWCNTs (free from MWCNTs)

were selectively obtained from camphor CVD at low pres-

sures (10–40 torr) where the camphor vapor pressure is

quite in tune with the low metal concentration.68 Unfortu-

nately, however, the quantity of such pure SWCNTs is still

in milligram order. Gram-order SWCNT synthesis from

camphor is still an open field of research.

5. CNT GROWTH MECHANISM UNDERELECTRON MICROSCOPY

As briefly outlined in Section 3, most widely-accepted

CNT growth model assumes that the hydrocarbon vapor

is adsorbed to the metal nanoparticles; and catalytically-

decomposed carbon species diffuse through the metal par-

ticle, and upon reaching supersaturation, precipitate out

in the form of seamless carbon cylinder. However, the

points of discord here are whether the metal is in solid

or liquid form, whether this diffusion is volume diffusion

or surface diffusion, whether the actual catalyst for CNT

growth is the pure metal or metal carbide, etc. Let us now

review some important in-situ electron microscopic studies

on these aspects.

5.1. Physical State of the Catalyst

The first effort to observe the carbon filament growth

process in-situ was made by Baker et al.8 By installing

a gas-reaction cell in the TEM specimen chamber, they

were able to perform carbon fiber growth in a tempera-

ture range of 600–1200 �C at different gas pressures up to

225 torr (maximum), while the TEM column was main-

tained at sufficiently low temperature and pressure suitable

for electron microscopy. For acetylene decomposition on

nickel catalyst supported on silica and graphite supports

at 600 �C, they clearly observed that the metal particles

changed it shape and moved up with a trail of carbon

deposit (30–50 nm diameter). From the changing shape

of the metal particle during fiber growth, they assumed

that the catalyst was in liquid phase. The activation energy

calculated for this growth was nearly same as the acti-

vation energy of carbon diffusion in liquid nickel; hence

they suggested that carbon diffuses through the bulk metal

and the fiber growth rate is diffusion-controlled. Simi-

lar tip-growth process was observed with Fe, Co and Cr

catalysts.9 But in the case of acetylene decomposition on

bimetallic (Pt–Fe) catalyst, the catalyst was observed to

remain static on the substrate, while the carbon filament

went on growing up. This led them to enunciate base-

growth model.10 It was explained that strong interaction

between Pt–Fe and SiO2 substrate kept the metal particle

anchored to the substrate surface, and carbon precipitation

occurred from the free upper face of the particle. Temper-

ature and concentration gradients were thought to be the

main driving forces for the continued growth dynamics.

The filament growth was seen to be ceased when the par-

ticle was fully covered with the carbon cloud, but it could

be re-activated by exposure to either hydrogen or oxygen

at higher temperatures.8 Later, however, many scientists

reported base-grown CNTs from Fe and Co catalysts on Si

and SiO2 substrates.7�60�146�147 This indicates that the same

set of hydrocarbon, catalyst and substrate may act differ-

ently in slightly different experimental conditions (temper-

ature, pressure, etc.).

In 1984 Gary Tibbetts explained why catalytically-

grown carbon nanofibers were tubular.148 Because the sur-

face free energy of the (002) basal plane of graphite is

exceptionally low, the free energy required for a filament

growth is minimum when graphite is in the form of a

seamless cylinder circumfering the metal. And the inner

core is hollow because inner cylindrical planes of small

diameter would be highly strained, energetically unfavor-

able to form. He also explained the CNT growth mech-

anism with a vapor–liquid–solid (VLS) model, originally

formulated for Si, Ge whiskers and many other crystals.149


REVIEW


Fig. 4. Melting temperature of selected metals as a function of particle

diameter. Reprinted with permission from [57], A. Moisala et al., J. Phys.:Condens. Mater. 15, S3011 (2003). © 2003, IOP Publishing Ltd.

Although this model is convincing and acceptable to a

great extent, it is often doubted how Fe, Co, Ni etc (normal

melting point ∼1500 �C) could be in liquid state within

600–900 �C, the growth temperature of typical CNTs in

CVD. Here it is important to note that the melting point

of nanoparticles below 10 nm falls abruptly (Fig. 4). For

instance, a 5-nm Fe and Co particle can melt at about

850 �C and 640 �C, respectively. These values (melting

points) fall on the border line casting the probability of

50–50 for both solid and liquid states of the metal. So it

is still hard to say on the metal’s physical state authorita-

tively. However, recalling that hydrocarbon decomposition

on metal surface is an exothermic reaction, it is likely that

the extra heat generated during hydrocarbon decomposi-

tion helps metal liquefaction to some extent. Hence the

(a) (b) (c) (d)

(e) (f) (g) (h)

Fig. 5. In situ HRTEM image sequence of a growing carbon nanofiber. Images (a–h) illustrate one cycle in the elongation/contraction process.

Drawings are included to guide the eye in locating the positions of mono-atomic Ni step-edges at the graphene–Ni interface. Scale bar = 5 nm.

Reprinted with permission from [153], S. Helveg et al., Nature 427, 426 (2004). © 2004, Nature Publishing Group.

opinion of active catalyst being in liquid phase wins, as

reported by many scientists for SWCNT growth.150�151 But

then what about the case of MWCNTs which usually grow

on bigger (>20 nm) metal particles? Bigger particles must

be in solid phase; and in turn, MWCNT would involve a

different growth mechanism than that of SWCNT!!

Another reasonable disagreement between the SWCNT

and MWCNT growth is on the existence of tempera-

ture gradient inside the metal catalyst. Baker’s explana-

tion of temperature-gradient driven fiber growth might be

applicable to MWCNTs which involve big catalyst par-

ticles. In the case of SWCNTs, however, it is very hard

to imagine a significant temperature gradient within a

particle of 1–2 nm. Hence SWCNT growth must be driven

by the carbon concentration gradient during the process.

Ding et al. have reported a molecular dynamics study of

SWCNT growth without temperature gradient which sup-

ports this view.152

5.2. Mode of Carbon Diffusion

Another highly-debatable question is whether the so-called

diffusion of carbon species through metal particle is sur-

face diffusion or bulk (volumetric) diffusion. Endo’s group

who extensively carried out benzene decomposition on

iron catalyst at 1100 �C,12 argued that hollow fiber could

form only by surface diffusion on the metal particle, as

earlier proposed by Baird et al.6�7 In 2004, a Danish group,

Helveg et al., succeeded in observing MWCNT growth

from methane decomposition at 500 �C on Ni catalyst

in a high-resolution TEM.153 They noted that, throughout

the growth process, the nickel cluster remained crystalline

with well-faceted shapes (Fig. 5). The graphite layers were


REVIEW


found to grow as a consequence of dynamic interaction

between carbon and nickel atoms. ‘Surface atoms’ of the

nickel cluster moved up and down, in and out (continu-

ously changing the metal’s surface texture) as if they were

knitting a graphene sheet out of the surrounding carbon

atoms. The nanocluster shape was periodically changing

its shape from spherical to cylindrical to align the graphene

layers around them. The authors proposed that the mono-

atomic steps on the cluster boundary played a key role

in anchoring carbon atoms and weaving the graphene net-

work. This observation reveals that the catalyst cluster is in

solid phase and the carbon diffusion is a surface diffusion

around the catalyst.

Later, Raty et al. reported a molecular dynamics simula-

tion study of the early stages of SWCNT growth on metal

nanoparticles.154 They showed that carbon atoms diffuse

only on the outer surface of the metal cluster. At first,

a graphene cap is formed which floats over the metal,

while the border atoms of the cap remain anchored to the

metal. Subsequently, more C atoms join the border atoms

pushing the cap up and thus constituting a cylindrical wall

(Fig. 6). In 2007, Robertson’ group also reported similar

findings by in-situ TEM observation of CNT growth.155

These evidences also explain the general experience that

small nanoparticles are crucial for SWCNT formation.

Small metal clusters (1–2 nm) have steep sharp edges

(atomic steps); hence they possess high catalytic activity

and are capable to form high-strain SWCNTs. With the

increasing cluster size the sharpness of the atomic steps at

the boundary decreases and so does their catalytic activ-

ity. Therefore, bigger metal clusters (5–20 nm) form less-

strained MWCNTs. Too big clusters (viz. 100 nm) acquire

almost spherical boundary with no sharp steps; that is why

they do not form CNTs at all.

Quite intriguingly, however, two months after Hoff-

mann’s report,155 Terrones and Banhart group reported an

exciting observation of CNT formation in an HRTEM by

simply holding a metal-encapsulated MWCNT at 600 �Cunder electron beam (300 kV) for 90 min.156 Carbon atoms

(i) (ii) (iii)

Fig. 6. Schematic representation of the basic steps of SWCNT growth

on a Fe catalyst, as observed in ab initio simulations. (i) Diffusion of

single C atoms (red spheres) on the surface of the catalyst. (ii) Formation

of an sp2 graphene sheet floating on the catalyst surface with edge atoms

covalently bonded to the metal. (iii) Root incorporation of diffusing single

C atoms (or dimers). Reprinted with permission from [154], J. Y. Raty

et al., Phys. Rev. Lett. 95, 096103 (2005). © 2005, American Physical

Society.

(a)

(b)

(c)

Fig. 7. In-situ observation of CNT growth under HRTEM. (a) Electron

beam knocks some carbon atoms from the MWCNT side walls into the

encapsulated metal cluster. (b) The metal cluster reshapes its flat cross

section into a convex dome and a carbon cap appears over the dome.

(c) At the base of the metal dome, atomic steps develop and new MWC-

NTs emerge coaxial to the original MWCNT. [Courtesy: M. Terrones]

from the side walls (the existing graphite layers around

the encapsulated metal) got injected into the metal bulk

and emerged in the form of new SW, DW and MWC-

NTs of smaller diameters coaxial to the original MWCNT

(Fig. 7). Such a prolonged observation of the CNT-growth

dynamics (atom-by-atom) clearly evidences bulk diffu-

sion. Nevertheless, we should note that this observation

was an exclusive case of rearrangement of the carbon-iron

ensemble inside a constrained nanoreactor (the original

MWCNT) under high-energy electron-beam irradiation, a

situation far away from usual CVD conditions. Hence such

bulk diffusion cannot be conceptualized as a general CNT

growth mechanism.

In the context of the changing metal shape during

CVD, it is pertinent to mention another aspect of the

CNT growth. Many a time we encounter CNTs with their

graphene layers inclined to the tube axis (herringbone or

stacked-cup structure). It is puzzling to think how they

form. Keeping in mind that graphite layers grow prefer-

entially on selected crystal planes of metal, this can be


REVIEW


understood as follows. The shape of the catalyst metal

cluster acts as a template for the surrounding graphene lay-

ers. Nanoclusters (say, 10–20 nm Ø) under suitable ther-

modynamic conditions tend to form an elongated cylin-

drical shape (viz., 3 nm Ø) so that CNTs grow with

the graphene layers parallel to the tube axis. Under cer-

tain (different) thermodynamic conditions the metal clus-

ters tend to become pear-shaped, giving birth to graphene

layers parallel to their inclined facets. This usually hap-

pens with bigger clusters (say, 100 nm) or for alloy

catalysts.157 When it comes to explain how such open-

edged nanographenes are energetically stable, scientists

suggest that those dangling bonds at the edges of the

stacked graphite platelets are stabilized with the hydro-

gen atoms expelled from the hydrocarbon (or from the H2

supply).158 Such a fiber is known as graphite whisker, and

many people do not consider that as a CNT.

5.3. Chemical State of the Catalyst

Another frequently debated point in the CNT growth

mechanism is about the chemical state of the active cat-

alyst. Most common concept is that the starting cata-

lyst material (pre-deposited on substrates) is usually in

oxide form. Even if we deposit fresh metal nanoparti-

cles on a substrate, the nanoparticles are quickly oxidized

when exposed to oxygen during the substrate transfer

to the CVD reactor. During CVD, hydrogen gas is sup-

plied to reduce the metal oxide into pure metal upon

which hydrocarbon decomposition and subsequent diffu-

sion leads to the CNT growth. Even when no hydrogen

is supplied externally, the hydrogen atoms liberated from

the hydrocarbon decomposition on the catalyst surface are

likely to serve the same. However, there are many con-

flicting reports right from the early-stage CVD experi-

ments. Baker and many others proposed that pure metal

is the active catalyst,159�160 while Endo and many others

detected the encapsulated particles (in the CNTs) to be

iron carbide.12�161�162

Among recent reports, Yoshida et al. performed atomic-

scale in-situ observation of acetylene decomposition on Fe

catalyst at 600 �C and 10−2 torr.163 Both SWCNT and

MWCNT were clearly observed to be growing from metal

particles rooted on the substrate as in base-growth model;

however, the metal’s shape was slightly fluctuating. Elec-

tron diffraction analysis of the metal clusters in each frame

was reported to match with that of iron carbide in cemen-

tite (Fe3C) form. Accordingly, the authors concluded that

the active catalyst was in fluctuating solid state of iron car-

bide; the carbon diffusion was volumetric; and all layers of

the MWCNTs grew up simultaneously, at the same growth

rate. However, latest in-situ electron microscopy and XPS

analysis by Wirth et al. emphatically advocate that the cata-

lyst exists in pure metallic form: right from the CNT nucle-

ation to the growth termination.164 When the CNT growth

ceases due to catalyst poisoning with excess carbon, that

supersaturated metal–carbon ensemble crystallizes in car-

bide form upon cooling.162 Confusion persists because the

lattice constants of iron and iron carbide are very close;

and for nanoparticles, some distortion in the lattice con-

stants is likely due to the small-size effect. These diversi-

ties of observation unambiguously reflect that we have not

matured enough to understand the world inside the nano-

tubes. Hence the research must go on in search of the fact.

6. MASS PRODUCTION OF CNTs

6.1. Mass Production of CNTs fromConventional Precursors

Since CVD is a well-known and established-industrial

process, CNT production by CVD is easy to scale

up. Several research groups translated their laboratory

methods to large scale. Smalley’s group developed the

high pressure carbon monoxide decomposition technique

(known as HiPco) for mass production of SWCNTs.29 In

this technique, iron pentacarbonyl catalyst liberates iron

particles in-situ at high temperature, whereas the high-

pressure (∼10 atmosphere) of carbon monoxide enhances

the carbon feedstock many folds, which significantly

speeds up the disproportionation of CO molecules into C

atoms and thus accelerates the SWCNT growth. Reported

yield of HiPco process is ∼0.45 g/h.165 The product is

commercially available at Carbon Nanotechnologies Inc.

(USA), which is reported to have a production capac-

ity of 65 g/h.166 Dai’s group has also scaled up SWCNT

production from methane pyrolysis over Fe–Mo bimetallic

catalyst supported on sol–gel derived alumina-silica multi-

component material. Their process yields ∼1.5 g SWCNTs

over 0.5 g catalyst hybrid as a result of 30 min CVD.167

Maruyama’s alcohol CVD technique45 is also adopted by

Toray Industries Inc. (Japan) with a reported capacity of

15 g/h; while Hata’s technique53 at AIST (Japan) can pro-

duce ∼100 g/h.166

As for MWCNTs, Endo’s group developed his method

of benzene pyrolysis on iron catalyst15 into a continuous

process for mass production. In his process metal catalyst,

benzene, and Ar/H2 gases are fed from the upper end of

a vertical furnace, and the resulting CNTs are collected

from the lower end. CNT growth occurs while the catalyst

particles are falling down (floating) through the furnace

at 1100 �C.168 The product is commercialized by Showa

Denko KK (Japan) which is reported to have a produc-

tion capacity of 16 kg/h.166 Nagy’s technique of acety-

lene CVD on various porous materials was also brought

up to industrial level by Nanocyl (Belgium) producing

∼1 kg/h MWCNTs.67�69 Fei’s group developed a nano-

agglomerate fluidized-bed reactor (1 m long and 25 cm

Ø quartz cylinder) in which continuous decomposition of

ethylene gas on Fe–Mo/vermiculite catalyst at 650 �C can

yield up to 3 kg/h aligned MWCNT bundles.169 Apart


REVIEW


from these university-venture-business associates, many

other companies are also there in CNT business. To name

some, Hyperion Catalysis International, Inc. USA (8 kg/h),

Nano Carbon Technologies Co. Ltd. Japan (5 kg/h), Sun

Nanotech Co. Ltd. China (0.6 kg/h), Shenzhen Nano-

Technologies Port Co. Ltd. China (5 kg/h).166

As per WTEC survey report,166 the consolidated CNT

production capacity of the world today is∼300 tons MWC-

NTs and ∼7 tons SWCNTs per year, while their expected

demand would exceed 1000 tons/year in the near future.

The present price of commercially available MWCNTs is

∼$1/g, while that of SWCNTs is ∼$100/g. At this rate,

CNT-based products would be about 10 times costlier than

prevalent products. Hence the scientists and engineers have

a great responsibility to develop more cost-effective pro-

duction techniques to bring down the prices to $100/kg and

$10,000/kg for MWCNTs and SWCNTs respectively.

A good sign is that CVD-based CNT technique is pro-

gressing fast, and innumerous companies are emerging.

However, a common problem of mass-produced CNTs is

that their purity decreases with the increasing yield. CNT

properties are easy to control in small reactors, as used

Before CVD (0.6g)

After CVD (6.6g)

Fig. 8. Photographs of the zeolite bed before and after CVD; and TG analysis of as-grown CNTs.

in academic laboratories. When the same technique is

transferred to big reactors, purity goes down and diame-

ter distribution broadens drastically. This problem warrants

that we must find more simple, more refined technique

which could be translated to large scale with the same

control. Moreover, in view of the growing environmen-

tal concern and increasing CNT demand, efforts should

be made to develop a sustainable technology. Rapidly

diminishing fossil fuel, alarms that methane-, acetylene-,

benzene-based CNT technology would not be sustainable.

Hence we must explore regenerative, renewable materials

for mass production of CNTs.

6.2. Mass Production of CNTs from Camphor

Researchers at Meijo University have reported gigasgrowth of CNTs from camphor.119 In a simple CVD reac-

tor (1 m long and 55 mm Ø quartz tube in a horizontal split

furnace), 30-min-CVD of 12 g camphor over 0.6 g Fe–Co-

impregnated zeolite powder at 650 �C yields 6.6 g CNTs

with an as-grown purity of >91% (Fig. 8). The volume

of the zeolite bed before CVD is 1.5 mm, which inflates


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to 150 mm owing to a gigantic CNT growth (10,000%).

Hence the authors call it gigas growth. In this process,

camphor-to-CNT production efficiency is >50%, which is

much higher than that reported from any CNT precursor in

an academic laboratory set-up. Here it is to be noted that

the carbon content of camphor is approximately 79 wt%.

So, with respect to the carbon content in the feed stock,

net carbon-to-CNT conversion efficiency comes out to be

61 wt%. There are plenty of reports of large-scale CNT

synthesis; but most of them express the yield either on the

basis of TEM observation, or wait gain relative to the cata-

lyst. From industrial point of view, the product yield must

be quoted with respect to the raw material used. CNT lit-

erature largely lacks this figure of merit. To compare cam-

phor’s efficiency with that of other precursors, CVDs of a

few liquid precursors were carried out on the same set up,

in the same conditions. CVD of 12 g ethanol and benzene

yielded about 2 g and 3 g CNTs, respectively; much lower

than the camphor case (6 g). Here it is arguable that every

precursor has its own set of best conditions. Ethanol or

benzene cannot give their best yields at the condition opti-

mized for camphor. So, more systemic study is required

to compare different precursors. Recently, Montoro et al.

carried out a comparative study of several alcohols and

ketones as a CNT precursor over Mn–Co-impregnated zeo-

lite support at 600 �C.170 Acetone was found the best pre-

cursor (better that ethanol). This report is in agreement

with our experience that camphor (a member of ketone

family) stood the best. More recently, Musso et al. also

investigated thermal decompositions of camphor, cyclo-

hexanol and ethanol at 900 �C using ferrocene catalyst;

and found that camphor gave the highest CNT yield with

best crystallinity.171 On the other hand, Das et al. studied

the effect of feedstock and process conditions on CNT syn-

thesis from several aromatic hydrocarbons (unfortunately,

camphor was not there).172 They systematically calculated

the CNT yield with respect to the feedstock, and found

that carbon-to-CNT conversion efficiency ranged within

20–39%. On that scale, camphor scores very high (61%).

The high CNT-production efficiency of camphor

(C10H16O) may be attributed to several factors.

(i) Cage-like carbon structure offers ease of transforma-

tion into fullerene and CNTs.

(ii) Ring-based CNT-growth process is supposed to lead

to higher growth rate.

(iii) Carbon-rich precursors have shown higher CNT

yields.

(iv) Abundance of hydrogen and presence of oxygen in

camphor may have a good coordination in reducing metal

oxide and etching amorphous carbon.

However, more-elaborated experiments and theoretical

supports are required to establish these reasons. Enthusias-

tic researchers are encouraged to take up these challenges.

6.3. Camphor versus Conventional CNT Precursors

As compared to conventional CNT precursors such as

CH4, C2H4, C2H2, C6H6, camphor (C10H16O) is carbon-

rich, hydrogen-rich, and oxygen-present. It is thought that

the bi-cyclic cage-structure of camphor plays a vital role

in such an efficient CNT growth. Most probably, the

hexagonal and pentagonal carbon rings of camphor con-

struct CNTs as a basic building block: without break-

ing into atomic carbon. Researchers often use to mix a

small amount of hydrogen gas in the carrier gas to reduce

metal oxide into pure metal catalyst. Abundance of hydro-

gen in camphor serves this purpose to a great extent

and eliminates the need of additional hydrogen mixing

in the carrier gas. Moreover, the oxygen atom present in

camphor molecule helps oxidizing amorphous carbon in-situ, as proposed by Maruyama et al.45 Thus every atom

of camphor has a positive role towards CNT synthesis.

A probable growth mechanism of CNTs from camphor

was first reported in 2003.117 Later, a Chinese group car-

ried out in-situ mass spectroscopy of benzene CVD and

supported the ring-based CNT growth hypothesis.173 In

particular, low-temperature camphor CVD (650 �C) rulesout the possibility of spontaneous (uncatalyzed) decompo-

sition of camphor vapor in the reactor, and results in CNTs

free from amorphous carbon. There is no trace of carbon

deposit in any portion of the quartz tube except on the

zeolite bed.

6.4. Environment-Friendliness of Camphor CVD

The United States Environmental Protection Agency has

formulated 12 Principles of Green Chemistry that explain

what green chemistry means in actual practice.174 Waste

prevention, atom economy, energy efficiency and renew-

able feed stock are most vital points for an industrial

process to be environmentally benign. Using those prin-

ciples as a protocol, we can evaluate how environment-

friendly this technique is. It is straightforward that the

higher is the yield the lesser is the waste. Exhibiting high-

est CNT-production efficiency, camphor complies with the

waste-prevention rule significantly. Moreover, as explained

above, every atom of camphor plays a positive role in

CNT synthesis, which accounts for maximum carbon-to-

CNT conversion efficiency. This is a good example of

atom economy. Further, by virtue of a low-temperature and

atmospheric-pressure CVD process, the energy require-

ment of this technique is very low (as compared to high-

temperature low-pressure CVD-CNT processes). Hence

certainly it is an energy-efficient method. And last but not

the least, the main raw material—camphor—being an agri-

cultural product, is absolutely a regenerative feedstock; so

there is no danger of depleting a natural resource. Thus,

camphor-based CNT synthesis technique complies with

the principles of green chemistry to a great extent. This

work has attracted the attention of industrial ecologists.175


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6.5. Industrial Production of CNTs from Camphor

Camphor CVD has been successfully scaled up to industrial

production of MWCNTs at Meijo Nano Carbon Co. Ltd.,

Japan. A big quartz tube (2 m long and 0.25 m Ø) installed

in an especially-designed rotary kiln (Takasago Co. Ltd.,

Japan) is used as a CVD reactor. Camphor and catalyst-

loaded zeolite powder are simultaneously fed in the reac-

tor from the left and right ends, respectively, through

two speed-controllable feeders. The quartz tube is slightly

inclined and keeps on rotating about its axis, so that the

zeolite powder falling on the right end of hot zone slides

down the slope; and by the time it reaches the left end of

the hot zone, CNTs would have efficiently grown onto it.

As-grown CNTs safely fall down into a reservoir at the

left end. The system can run continuously for hours. Aav-

erage production rate is about 1 kg/h and the purity of

as-grown MWCNTs is about 90%. Due to extremely low

price of camphor ($10/kg), the expected production cost of

as-grown CNTs is very low ($100/kg).

7. EXISTING CHALLENGES ANDFUTURE DIRECTIONS

In the foregoing sections, we raised several questions on

the role of precursor, catalyst, catalyst support, growth

mechanism and mass production, and indicated possible

directions. In addition to those basic issues, other growth-

related challenges are briefly outlined below.

1. Researchers have succeeded in minimizing the diameter

distribution of SWCNTs up to some extent.176 However,

synthesis of SWCNTs of a given diameter is yet to be

achieved.177 It would be possible only when we have the

catalyst particles all of exactly the same diameter (say,

0.5 nm).

2. Chirality control is even more challenging. Re-growth

from ordered arrays of open-ended SWCNTs may help

up to some extent.178 Alternatively, we have to develop

proper separation methods that could first sort out CNTs

according to metallic or semiconducting tubes and then

select tubes of certain chirality.

3. In MWCNTs, control on the number of walls is another

big challenge. Synthesis of thin MWCNTs (3–6 walls) is

a better choice than thick MWCNTs.179�180

4. Growth of isolated CNTs has not yet reached a mature

stage. There are indications that even a single CNT does

not possess the same diameter and chirality over the entire

length. How can we solve it?

5. Researchers have succeeded in growing CNTs from

almost all metals.181 However, we do not know how differ-

ent metals affect physical, chemical, electronic and optical

and magnetic properties of as-grown CNTs. If we could

discover any correlation between the type of the metal

used and the property of the CNT grown, we would be

able to grow CNTs of selective properties.

6. What is/are the determining steps in CNT nucleation

and growth? Having known these steps, the CNT growth

rate could be increased for mass production.182

7. The exact role of H2, O2 and H2O in CNT growth is

yet to be clarified.183 Simultaneous presence of reducing

as well as oxidizing agents in the reaction zone makes it

ambiguous whether amorphous carbon is etched by atomic

hydrogen, oxygen or water. Are they really essential?54

8. Researchers have succeeded in bringing down the CNT

growth temperature to ∼400 �C in low-pressure CVD.184

However, low-pressure CVD greatly reduces the growth

rate and yield. Low-temperature CNT growth must be

devised at atmospheric pressure for high yields of CNTs.

9. Many technological applications are looking for room-

temperature CNT growth which is still a dream, so far as

thermal CVD is concerned.

10. Mass-produced CNTs usually contain catalyst par-

ticles or support materials as impurity. Post-deposition

purification greatly reduces the CNT quality and final out-

put. More thoughts should be given to achieve high-purity

in as-grown state.

11. CVD-grown CNTs (especially low-temperature

MWCNTs) have poor crystallinity. With a suitable com-

bination of different catalysts, it should be possible to get

better-crystallinity CNTs.

12. Recent metal-free oxygen-assisted CNT growth is a

breakthrough.99–101 It must be scaled up to mass produc-

tion of high-purity CNTs.

13. Carbon–metal phase diagram needs to be recon-

structed, especially for 1–5 nm range relevant to CNT

growth.185

14. All extraordinary properties of CNTs are predicted for

atomically-perfect CNTs. To make those predictions true,

it is of prime importance to develop new techniques to

monitor and remove defects in-situ.15. Lack of quality control and assessment of CNTs syn-

thesized by different groups by different methods does not

allow us to get correct product details. Analytical sampling

of CNTs obtained from different sources at an authorized

standard laboratory would reveal exact merits and demer-

its of different techniques, which would in turn help us

explore combinations of techniques toward higher-yield,

higher-purity and lower-cost mass production.

What we have mentioned above are only growth-related

challenges. There are many application-related challenges

whose mention is beyond the scope of this review.

8. CONCLUDING REMARKS

In 2000 Rick Smalley said about CNTs, “Buckytubes are

in high school now.” After 10 years of his statement, we

feel that CNTs are now in university; they have not yet

graduated and joined their jobs out. As we have seen in

the foregoing sections, despite great progresses over the

years, there are many basic issues concerning the CNT


REVIEW


growth mechanism which are still not clear. Contradictory

observations of CNT growth under electron microscopy by

different groups suggest that the mechanism is extremely

sensitive to each parameter such as carbon precursor, metal

catalyst, particle size, temperature, pressure. Even a minor

change in any of these parameters leads the growth in crit-

ically different directions. Catalysis is the main stem of

CVD-CNT technique; and it seems that we have not yet

utilised the best of catalysis in this field. Nano-catalystmaterials are needed to be developed and investigated in

more detail. In principle, with the use of a suitable cata-

lyst, the CVD temperature can be brought down to room

temperature. By identifying the growth-limiting steps it

should be possible to control the diameter and chirality

of the resulting CNTs. To comply with the environmental

concerns, renewable materials should be explored as CNT

precursors. In view of the expected giant demand of CNTs

in the near future, industrial production of CNTs should

be carried out with far-sighted thoughts for long-term sus-

tainability. Fossil-fuel based CNT-production technology

would not be sustainable. The unanswered questions about

growth mechanism and the existing challenges concerning

the growth control will keep the CNT researchers engaged

for a long time. In quest for knowledge, in pursuit of per-

fection, and for contributing to science and society, the

CNT research must go on.

Acknowledgments: This work was financially sup-

ported by the Japan Society for the Promotion of Science

(Grants-in-aid for Scientific Research), the Ministry of

Economy Trade and Industry (Regional Resources R&D),

and the Ministry of Education, Culture, Sports, Science

and Technology (Regional Innovation Creation R&D).

The authors are grateful to Dr. Toshio Kurauchi (For-

mer Vice President, Toyota Central R&D Labs Inc.),

Mr. Takeshi Hashimoto (President, Meijo Nano Carbon

Co. Ltd.), Mr. Akira Kagohashi (Managing Director,

Takasago Industry Co. Ltd.) and Mr. Hirotaka Masuoka

(Director, Masuoka Ceramic Materials Co. Ltd.), whose

joint efforts drove our work from laboratory to industry.

Mukul Kumar thanks Dr. Debabrata Pradhan for his great

help on the references.

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Received: 29 January 2010. Revised/Accepted: 1 February 2010.


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